The electric utility industry strives to efficiently provide electric power while minimizing the impact that electrical generation has on the environment. One specific point of concern is reducing harmful exhaust gas emissions from power plants.
Power plants produce dangerous combustion gases, such as NOx (oxides of nitrogen like NO and NO2), which are exhausted as a by-product of electric generation. The combustion or flue gases are carried by large ducts or flues through treatment systems intended to reduce the NOx emissions. One commonly employed treatment process, called selective catalytic reduction (SRC), reduces NOx emissions by injecting an ammonia mixture into the combustion gases and passing the combined constituent gas, combustion gas mixed with the ammonia mixture, over a catalyst. The catalyst reacts with the harmful gas changing it into harmless gas comprised of nitrogen related compounds, thus reducing or eliminating the NOx emissions.
When the combustion gases are uniformly mixed with the ammonia mixture and passed over the catalyst within a specific temperature range, the catalyst is highly effective at reducing NOx emissions. However, a uniform constituent gas mixture is difficult to achieve given the volume of combustion gas which must be uniformly mixed within the large ducts. These ducts range in shape, such as rectangular and oval, and size, but often have passageways of 20 feet by 40 feet or more.
Frequently, the constituent gas flowing within the ducts develops small channels containing high concentrations of the various flue gas constituents (CO, CO2, NOx, for example) and the injected ammonia, or rope flows, while the constituent gas throughout the remaining duct cross section will contain low concentrations of NOx making proper mixing with ammonia prior to the SCR most difficult. When such inhomogeneous constituent gas is passed through the catalyst, the rope flow zones exit the catalyst with high levels of flue gas constituents and/or ammonia that were not reacted by the catalyst due to the improper mixing of flue gas and ammonia. The remaining combustion gas with low concentrations of NOx will be catalyzed by the SCR, but the ammonia will be under utilized and emitted into the atmosphere at greater than acceptable concentration.
The remedy has generally been to inject more ammonia mixture into the combustion gas in the ducts to reduce the NOx emissions. While this does lower NOx, increasing the ammonia concentration is costly, inefficient, and results in increased ammonia emissions.
Another approach has been to place obstacles into the ducts to disrupt the flow of combustion gas in order to achieve an improved mixture of constituent gases. However, such obstacles create only turbulent flow of the combustion gases and provide only minimum improvement of the mixture of constituent gases. Additionally, the turbulence inducers generate resistance in the ducts which reduces the efficiency of the flow of combustion gases through the ducts and increases the load on the fan systems that move the combustion gases through the ducts.
Thus, a need exists for an improved system and method for mixing the stream of combustion gases and injected ammonia mixture into a uniform mixture of constituent gases. It is to such a gas stream mixing system and method that the present invention is directed.